Back in part one of our new GMPP LS3 testing ("Late Crate Update," Oct. '10), we subjected a factory-fresh LS3 crate motor from GM Performance parts to a series of performance upgrades, including a new Comp cam and GMPP CNC-ported cylinder heads. Applied to the already impressive powerplant, the heads and cam package improved the power output of the LS3 from 492 hp at 5,900 rpm and 484 lb-ft at 4,700 rpm to 569 hp at 6,500 rpm and 522 lb-ft at 5,200 rpm. As impressed as we were with the results of the head and cam package, we couldn't help but wonder if there was even more power available from the combination. Obviously, more power could be had from increased compression or wilder cam timing, but we wanted our power gains to come with little or no cost to driveability, and we didn't want to run costly race fuel.

2/18Everyone was very eager to get all the hardware bolted on to this 417 stroker LS3. Would the increased displacement yield the power and torque gains we were looking for?

Enter the stroker kit-not just any stroker kit, but one from Demon Engines and Procomp that consists of a 4.00-inch, 4340 forged-steel crank and matching forged 6.20-inch connecting rods. These components were combined with a set of forged-aluminum slugs from Probe Racing to complete the rotating assembly. The flat-top pistons featured the required 4.070-inch bore and valve reliefs for our 0.600-inch-plus-lift Comp hydraulic roller cam. The combination of the good bore and 4.00-inch stroke produced a finished displacement of just below 417 ci.

That stroker assembly represented a jump in displacement of 41 ci. The 569hp, 376ci LS3 produced a specific output of 1.513 hp/ci. If we apply that specific output to our new 417ci engine, we see a potential of 630 hp. Unfortunately, the specific output of the 417 will not equal that of the smaller 376 because the larger motor will require wilder cam timing to do so. We had no intention of running a more aggressive cam profile, as the additional benefit of the increased displacement would be improved torque, idle quality, and driveability. The hike in displacement would effectively tame the cam timing. The combination of the effectively milder cam timing and increased displacement should shift the power curve lower in the rev range, but we still wanted the ability to rev the motor for maximum acceleration when the time came. We hoped the increase in displacement would work well with the large rectangular ports in the GMPP CNC heads. If it sounds like we had high hopes for our stroker LS3, we did.

3/18Originally, we subjected the GMPP LS3 crate motor to a few select bolt-ons. Running the GMPP CNC heads and Comp Xtreme Energy cam improved the power output of the LS3 from 492 hp and 484 lb-ft to 569 hp and 522 lb-ft.

The LS3 stroker short-block was machined by the good folks at L&R Automotive and assembled by Demon Engines. The aluminum LS3 block was bored to 4.070 to make room for the pistons.

4/18The first step was to disassemble the GMPP short-block and have it machined to accept a completely new rotating assembly from Demon Engines.

Prior to assembly, the Procomp crank, rods, and Probe pistons were given a precision balance job and assembled using Speed-Pro rings, Clevite bearings, and Fel-Pro gaskets, oil pump, and timing chain. It was necessary to clearance and space the factory windage tray to clear the stroker crank and rods. We installed the same Comp hydraulic roller cam that featured a 0.617/0.624-inch lift, 231/239 degrees of duration, and a 113-degree LSA using Comp hydraulic roller lifters. The CNC-ported GMPP heads were installed using Fel-Pro MLS head gaskets and ARP head studs. Finishing touches included a FAST LSXR intake and 102mm throttle-body, a set of 17/8-inch American Racing headers, and 65-pound injectors (the factory injectors will work at this power level with sufficient static fuel pressure).

Naturally, we were excited about the power output of our new LS3 stroker combination, but we subjected it to a computer-controlled break-in procedure using Lucas 5W-30 conventional oil and a new K&N oil filter. The FAST XFI management system was employed to optimize the air/fuel and timing curves, and Westech's Ernie Mena had the combination tuned to perfection in record time. The factory plugs were replaced with a set of performance plugs from E3 featuring Diamond Fire Technology. With the air/fuel dialed in near 13.0:1 and the total timing at 30 degrees, the 417 stroker thumped out peak numbers of 608 hp at 6,300 rpm and 570 lb-ft at 5,100 rpm. This easily eclipsed the 569 hp and 522 lb-ft produced by the stock-displacement LS3 run previously. Torque production from the 417 stroker exceeded 550 lb-ft from 4,200 rpm to 5,700 rpm. Compared with the modified LS3, the stroker combination produced 500 lb-ft 600 rpm earlier and carried that number 500 rpm later than the stock-displacement combo. Let's not forget that the stroker offered a gain of nearly 50 lb-ft measured peak to peak. Basically, the stroker did everything the stock-displacement motor did, just bigger and better. This will be one seriously stout street motor in an early Chevelle or Camaro, but we know our readers can't resist taking things to the next level. Check back with us next month a see what happens when we add a little boost to the equation.

5/18The block was machined to accept a 4.00-inch stroker crank from Procomp. The 4340 forged-steel crank was installed using the factory main studs, but ARP does offer a main-stud kit for LS motors.

Modified GMPP LS3 vs 417 Stroker
The graph tells the tale, as the 417 stroker offered significant power and torque gains through the entire rev range, increasing the power output of the LS3 from 569 hp and 522 lb-ft of torque to 608 hp and 570 lb-ft of torque. The gains were as high as 50 hp and 60 lb-ft of torque elsewhere in the curve, but the solid torque gain is something that can be enjoyed every time you even tickle the loud pedal. Not shown on the graph is the fact that the new combination improved the idle quality (by nearly 1 inch of vacuum at idle). Having all that extra torque means less need to rev the engine, though the stroker pulled hard to 6,500 rpm and beyond. We can't wait to see what happens when we add boost!